Aromatic-ring-layered polymers composed of fluorene and xanthene

INTRODUCTION Conjugated polymers show intriguing properties such as electrical conductivity,1–4 electroluminescence5–9 and chemical-sensing ability.10–12 Synthesis of new conjugated polymers is an important subject in polymer chemistry as well as in material chemistry, owing to their potential applications in optoelectronic devices. Generally, p-electrons are delocalized throughout the conjugated polymer backbone comprising of spand/or sp2-carbons. Recently, extensive studies have been conducted on conjugated polymers featuring the incorporation of main group elements. Various unique orbit interactions exist between the main group elements and p-conjugated carbon frameworks, for example, s–p (or s*–p*) conjugation13,14 for Si and p–p* conjugation15–17 for B. Furthermore, the properties of a new class of conjugated polymers that show through-bond conjugation and through-space conjugation in a polymer chain have been investigated. Through-space conjugated polymers were prepared by incorporating pseudo-para-18–24 and pseudo-ortho-substituted25 [2.2]paracyclophane into the conjugated polymer backbone, and their optical and electrochemical properties have been investigated in detail. Recently, pseudo-geminal-substituted [2.2]paracyclophane was used for the synthesis of through-space conjugated polymers, in which the p-electron systems fully overlapped with each other and showed a large Stokes shift.26 Polymers comprising layered p-electron systems have also attracted much attention as novel functional polymers.27 We have previously reported a new synthetic strategy for constructing aromatic ringlayered structures in a polymer main chain by using xanthene or naphthalene as the scaffold. This approach makes it possible to layer various aromatic rings in a single polymer chain; for example, cyclophane-,28,29 oligothiophene-30 and anthracene-layered polymers31 were synthesized. Expansion of the substrate scope and investigation of the newly developed aromatic ring-layered polymers are of importance in the field of the conjugated polymers. In this study, we used fluorene as the layered aromatic unit and xanthene as the scaffold. Fluorene compounds possess easy functionality, thermal stability and prominent emitting-property; thus, they are valuable emitters as well as molecular probes to determine the environment via fluorescence spectroscopy due to high-quantum efficiencies and clear vibrational spectra. The fluorene-layered polymers were synthesized by the Sonogashira–Hagihara coupling reaction. The synthetic procedure, characterization and optical properties of the polymers were investigated in detail.